What should I write in a memorandum on behalf of women in Saudi Arabia to the Secretary General of the United Nations?

Presumably, your memorandum is supposed to convince him to try to do something to improve the lives of women in Saudi Arabia. If so, you should point out to him all of the sorts of things that women are not allowed to do in that country. The two most famous examples are that women are not allowed to drive and that they must (for their entire lives) have a male guardian. This male guardian has to give his approval if the woman wants to do anything like traveling outside the country or even opening a bank account. You should point out that Saudi Arabia could serve as a model to other nations in the area if it would treat women more equally. Finally, you should tell the Secretary General what it is that you hope he and the UN will do to help.